Devices in which energy is absorbed by stretching a tension member against its material and form resistance are used in many areas of application, such as e.g. fall brakes in mountaineering or assembly work in the construction trade, as so-called “shock absorbers” for safety belts in automotive engineering, or as brakes in dynamically stressed supporting structures such as safety nets, rope barriers and the like.
Plastically deformable tension members are preferred here because with predominantly elastic deformability, as provided e.g. by non-overstretched springs made of spring steel or rubber straps, the kinetic energy absorbed was only temporarily stored and then the majority of it was returned to the braked body, which would set the latter in motion again.
With energy absorption a constant development of the braking force over the whole braking path is desirable because in this way the moving body is braked with constant negative acceleration and so is subjected to forces which remain uniform.
However, when subjected to tension, tension members in the form of elastically or plastically deformable helical springs do not have constant spring forces over the range of the spring. With a helical spring the spring force rises as the range of the spring increases because all of the convolutions are stressed simultaneously by the latter stretching with simultaneous reduction of the convolution diameter and increase in the pitch, and so constantly increase their resistance to further stretching.